This is what Bushmaster's faq had to say on the situation. All three questions in this thread are covered in it, plus much more. I'm not saying that I agree with everything they say, notably that the fluted barrel will "last longer", but it's a good place to start, and in the very least fan the flames.
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Q. How much difference is there between the use of the 20" and the 24" barrel. I like varmint shooting, and someday would like to compete in DCM matches. Is there a notable difference?
A. With factory ammo, the difference between a 20" barrel and a 24" barrel boils down to about 100 fps (feet per second - velocity). If you handload, you have a greater diversity of loads/powders that will function reliably. The 24" bbl. really does not give you anything extra over a 20" bbl. until you get out to about 400 yards where the extra velocity starts to make a difference. And, the longer barrels do very well with the heavier bullets (69 - 72 grain) at those ranges. If your specific interest is varmint hunting or long range target shooting, you may well benefit from a longer barrel. "DCM" Competition gets very specialized (and very long distance - up to 600 yards) - and when you get to that level, you'll probably want an extra heavy competition barrel - however, "DCM" Competition rules require the use of a 20" barrel. To accurize your DCM use rifle, we also offer 1/4 Minute of Angle Rear Sights with Micro-peeps; DCM legal trigger jobs; "free-floater" type handguard and buttstock counterbalance weights. We have all of these competition parts, so give us a call (1-800-998-SWAT) or see the Competition Parts Section of the Internet Catalog.
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Q. Does fluting make that big a difference in accuracy?
A. Fluting (machining lengthwise grooves into the outer surface of the barrel) increases the surface area of the barrel - thereby allowing increased radiational cooling of the barrel mass. This process also adds to the "stiffness" of the barrel which enhances its accuracy. A third benefit of fluting is that the barrel is lighter after machining removes the steel in those lengthwise grooves. A fluted barrel will last longer, shoot straighter, and is considerably lighter than a standard heavy barrel. The accuracy gains would be most noticeable at longer distances or to the competition shooter, but for only $50 additional for most Bushmaster barrels, many feel that the benefits of fluting are well worth the cost.
Q. Is a Stainless Steel barrel any better than a chrome lined?
A. Stainless steel is better at preventing erosion than regular 4140 steel, but we use mil. spec. 4150 ordinance steel. Then, our barrels are chrome lined and a chrome lined barrel will easily out-last a stainless barrel. A very good article on barrel manufacturing can be found in the '96 Shooter's Bible (Pg. 33). A typical stainless barrel is made from 416 stainless steel and then broach rifled. This process has been around for about 100 years. Our chrome lined barrels are made from 4150 ordnance steel and then button rifled - a process that's been around for about 50 years. This same process has set virtually every record for the National Bench Rest Association (NBRA). Shilen, McMillan and Browning barrels all use the same process but not the same steel. Mil. spec. calls for 4150 steel - same as used in aircraft machinegun barrels and all military small arms barrels. It costs more but we think its well worth the price. The button rifling process work hardens the bore - making tough steel even tougher. Then, after the barrel is fully machined, it is chrome lined, making it even tougher yet - and virtually impervious to rust or erosion. This chroming process isn't like car bumper chroming. It actually welds each chromium molecule to the steel bore. This chrome lining is far more resistant to wear than a bare steel bore and it gives slightly increased velocity due to the lubricity ("slipperiness") of the chrome. And, you'll see less fouling and easier cleaning with a chrome lined barrel - all in all, a superior product.
http://www.bushmaster.com/faqs/barrels-accuracy.asp